x86: refactor ->enable_apic_mode() subarch methods

Only ES7000 has a real ->enable_apic_mode() method, the other
subarchitectures define it but keep it empty.

So mark the vector as NULL, extend the generic code to handle
NULL -setup_portio_remap() entries and remove all the empty
handlers.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/apic.c b/arch/x86/kernel/apic.c
index fcbcc03..9d6374d 100644
--- a/arch/x86/kernel/apic.c
+++ b/arch/x86/kernel/apic.c
@@ -1763,7 +1763,8 @@
 		outb(0x01, 0x23);
 	}
 #endif
-	enable_apic_mode();
+	if (apic->enable_apic_mode)
+		apic->enable_apic_mode();
 }
 
 /**